1. Field of the Invention
The present invention relates to mobile aerial towers and in particular to a fluid control system for selectively operating the various positioning elements such as the upper and lower booms and the turntable.
2. Description of the Prior Art
Mobile aerial towers are well known and generally comprise a truck mounted pedestal on which the tower structure is mounted. The tower structure conventionally comprises a rotatable platform or turntable mounted on the pedestal, an elongated lower boom pivotally mounted on the turntable at one end for swinging movement about a horizontal axis and an upper boom pivotally connected to the other end of the lower boom for swinging movement about a horizontal axis. To the opposite end of the upper boom is pivotally connected a personnel bucket or platform.
Hydraulic motors, often of the double acting piston type, are connected to the upper and lower booms and a suitable hydraulic motor is connected to the turntable to impart rotary motion thereto. In order to permit the tower structure to be positioned by the person carried in the bucket, a remote control unit is mounted therein. Prior art control systems for operating aerial towers of this type are disclosed in L. L. Myers U.S. Pat. Nos. 2,836,467; 2,946,196; 3,133,471 and 3,415,021.
In Myers U.S. Pat. No. 2,946,196 a remote control operator is disclosed wherein a single control lever is connected through suitable mechanical linkage to a plurality of pilot cylinders which in turn are operatively connected to the hydraulic motor control valve through hydraulic lines. As the control lever is moved in a certain direction, one of the pilot cylinders will be actuated thereby pressurizing the hydraulic fluid in its respective line so that one of the motor control valves will be thrown to admit fluid pressure to one side or the other of its respective motor. In this particular instance, a single fluid pump is employed which supplies pressurized hydraulic fluid to each of the motors by individual selectively actuated valves of the type described above.
Due to recent concern over fuel conservation and environmental protection, many mobile aerial tower users, such as utility companies, have turned to electrically operated towers. Typically, battery power is substituted for the gasoline engine and is designed to be charged while the truck is being driven to the work site. When the truck arrives, the engine is shut off and the tower is operated off the battery pack.
One problem with existing battery powered pumps, however, is that they are quite inefficient. When the pump motor is connected to the battery, the pump operates at full speed so that in order to obtain lower speeds, the use of a relief valve is necessary to divert the excess hydraulic pressure. Obviously, operating at only one speed is very undesirable from the standpoint of safety especially when the tower is utilized in work on high voltage power lines. Accordingly, most mobile aerial towers have booms and turntables which are capable of being operated at low, medium and high speeds.
When the tower is being operated at a slower speed, the control valve is moved only a short distance but the pump is still developing its rated pressure. Since only part of the pressure is being utilized, the remainder is passed through a relief valve and an anomalous situation occurs where there is greater battery drain at low speeds than there is at high speed. This is especially true when the bucket is being lowered where only 100 psi or 200 psi of hydraulic pressure out of a system capacity of 1500 psi is necessary because of the effect of gravity. The pump is still operating at full capacity, however, building up full relief pressure with the concomitant drain on the battery.
Of course, if more than one positioning element is being operated simultaneously, a greater percentage of the hydraulic pressure generated by the pump will be utilized. There is a limit to the number of elements which should be operated at one time, however, because of the difficulty in controlling the position of the bucket when rotation about a vertical axis and extension about two horizontal axes are occurring simultaneously. For this reason, it is desirable to limit the number of positioning operations which can occur simultaneously to two. For example, one of the booms and the turntable or, alternatively, the upper and lower booms could be safely operated simultaneously. Another safety problem which may occur is the operation of more than one positioning element at high speed.